I have a similar issue with ferrite 'balun' heating, although mostly
tested on 40 metres. I'm using an OSCFD (Windom) which has a main 4:1
balun at the feed point, but also has a sleeve 'balun' consisting of 8
ferrites wrapped in a plastic sleeve, about 3 feet from the feed point.
If I run the K3S at 100W, via the KPA500 in standby, then through the
KAT500 tuner, I see no problems.
On increasing the power to 200W by using the KPA500 in Operate and about
10W drive from the K3S, during a SSB 'over' the temperature of the
KPA500 rises and after a few minutes, the SWR readings on the KPA500 and
KAT500 start to rise. One of the Elecraft guys actually very kindly went
through my fault log and came to the conclusion that the fault is with
the antenna/feeder and I agree with him. What is strange is that if I
leave a solid carrier running at 200W, I can't easily reproduce the
issue and I wonder if its something being affected by peak excursions
with SSB?
When things do start to go wrong, I can usually perform a manual tune
with the KAT500 and the fault will then not (mostly) be apparent.
Anyway, I'm in the process of replacing the entire feeder with Westflex
103 and much bigger ferrites. I will also replace the main balun which
is a 400W device for a 1KW device, as I think that running the UK max of
400W may be pushing the spec of a 400W balun. Also, the balun itself or
connections to it may be faulty.
The point now is that I have discovered some damage to the outer
covering of the sleeve 'balun' which may be the result of ferrites
overheating or may be the cause of movement of the ferrites thus causing
instability. Either way, heftier devices seem to be the order of the
day.
Although the KPA500 temperature rise is somewhat expected, I feel it's a
bit higher over a short period than I would like and may be due to
reflected power.
73,
Alan. G4GNX
------ Original Message ------
From: "David Olean" <[email protected]>
To: "Peter Dougherty" <[email protected]>
Cc: [email protected]
Sent: 01/06/2020 13:25:18
Subject: Re: [Elecraft] NOT the feedline (was KPA-1500 faulting on 6m;
SWR issues)
Hello Peter
I suspect RFI as well. I do not have a KPA1500, so my advice is not worth much,
but the fact that the fault only occurs when a signal is being radiated sends
me a red flag. I had a similar problem here with my SPE amplifier power
output. I was measuring it on an LP-100A wattmeter. It turns out that 1.8 MHz
energy from the SPE amp radiated from my vertical antenna and was getting into
a new wire doublet antenna and coming back down the ladder line into the shack.
The ladder line was about 7 inches from the power coupler for the LP-100A. RF
caused the readings to go wacky and it looked like my amplifier was going
postal! The problem was high rf fields next to the power meter coupler.
Ferrite balun heating can be seen with a VSWR meter if you set the power at
some intermediate level, key the amp up with a steady carrier, and watch for
VSWR creeping up. Loose pins in coax can also be a problem. They can heat up
and disconnect or start arcing. Those problems do not typically show VSWR
creep. They happen all at once usually.
The KPA1500 works fine into a dummy load. (No RFI) That is a big clue. Good
luck and I hope you find the problem. Maybe an RF sniffer would detect the
ingress point?
73
Dave K1WHS
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